“X-Ray Sources” or “X-Ray Tubes” are the generating source of x-rays used in a wide variety of medical, industrial, and research devices, with many different sizes, configurations, and enclosures required by the large variety of machines that use them. However, all x-ray tubes operate by the same principle of bremsstrahlung, or braking radiation, as that of the very first sources of x-rays when x-rays were first discovered by W. C. Roentgen in 1895.
In these devices x-rays are produced through the interaction of high speed electrons with the atomic structure of a target material. In a typical x-ray tube configuration the electrons are emitted at the cathode though various means such as thermionic emission from a tungsten filament. Application of a potential difference between the cathode and the target, such that the target is electrically positive in regards to the cathode, shapes the electrons into a focused beam and accelerates the electrons to high velocities. Accelerating potentials of 150 kVp will result in electrons travelling at approximately two-thirds the speed of light. To avoid interaction with other gas atoms and molecules this process is performed in a high vacuum environment. When these energetic, high speed electrons strike the target material, which is part of an anode structure, they interact with the atoms of the target material. These interactions result in the deceleration of the high speed electrons and the release of energy. At best, only about 1% of the energy of the electrons is converted into x-radiation. The remaining energy transforms into heat energy. X-rays are generated in all directions and with a variety of energies ranging up to that of the accelerating electron. Only those x-rays traveling in the direction required for use exit the x-ray tube. The remaining x-rays are attenuated by the material of the x-ray tube housing.
Early x-ray tubes were derived from cathode ray tubes known as Crookes or Hittorf's tubes which were popular in the scientific community at the time of x-ray discovery in 1895. The Crookes cathode ray tube includes a sealed cylindrical glass tube, in which two electrodes are placed. One electrode, termed the cathode, was sealed in line with the main axis of the tube. One electrode termed the anode was placed off axis usually laterally in the cylindrical wall of the tube. When the tube was evacuated to a level of 0.01 mmHg and a sufficient electrical potential was applied between the electrodes, ionization of the residual gas in the x-ray tube would occur. The negative potential applied to the cathode caused positively charged gas ions to be accelerated to the cathode surface. These ions bombarded the surface of the cathode which caused the ejection of electrons. These electrons in turn were accelerated by the electric field down the axis of the tube and impacted the glass wall opposite the cathode thereby generating x-rays. The electrons eventually drained across the inside glass surface and to the anode electrode. Drawbacks of such devices included the rapid heating of the glass surface due to the poor efficiency of x-ray production.
These drawbacks led to the development of the ion x-ray tube in the first decade of the 20th century. An ion x-ray tube included a focusing cathode, one or two anode assemblies, and a means to regulate the internal vacuum level. The focusing cathode was metallic, usually aluminum, and had a concave, spherical shape that focused electrons onto a small area, called the focal spot, on the target mass. The anode was generally a thin refractory material such as tungsten or platinum brazed to a heavy mass of copper. The copper provided quick heat transfer away from the focal spot. Operation of the tube depended on the correct vacuum level. As a result of normal tube operation, the tube pressure would gradually be reduced, and methods for raising the pressure to support gas ionization in the tube were developed. These methods usually worked on the principle of diffusing gas through thin walled tubes of palladium or platinum. Methods to reduce the tube pressure, should it be too high, included intermittent operation of the tube with low tube currents.
Ion x-ray tubes were replaced with the introduction of the high vacuum, high voltage x-ray tubes, first introduced in 1913 by W. D. Coolidge. This included the basic design principles used by many modern high power x-ray tubes. The main advantages of the high vacuum, high voltage Coolidge tube include the elimination of gas ions, which cause erratic operation, and the independent control of the tube current and the applied potential. High vacuum x-ray tubes, or Coolidge tubes, operate on the principle of electron emission, such as that from a hot tungsten filament, located in the cathode assembly. As gas ionization is no longer required for the operation of the tube, a Coolidge tube typically operates in the range of 0.000001 mmHg or lower. To increase the quantity of x-rays generated, the filament is heated electrically, thereby increasing the emission of electrons which are then accelerated to the target. To increase the penetrating ability of the x-ray, the applied tube potential can be increased independent of the tube current.
Improvements in the thermal loading capability of the focal spot and therefore x-ray tube power were made in the Coolidge x-ray tube with the introduction of the line focus concept and rotation of the anode assembly. Gas ion tubes produce circular electron beams that, when impacting a target placed at 45 degrees to the normal of the electron beam, produce an x-ray focal spot of an apparent circular cross-section when viewed along the central ray of the tube, which is at right angles to the normal of the electron beam. To improve thermal loading and main image resolution, the high vacuum tube utilizes a filament coil to create an electron beam of rectangular cross-section. When this beam impacts a sloping target with a shallow target angle, the apparent focal spot size along the central ray will appear to be emitted from a much smaller area. An additional improvement was made with the introduction of a rotating anode assembly, to which the target material is attached. By spinning the target during an exposure, target loading is increased by the ratio of the focal spot width to the circumferential length of the target track thereby greatly increasing the power capability of the x-ray tube.